Why is the Gospel of John the most popular book for a Bible study?

"If the Bible were a mountain, the thirty-nine books of the Old Testament lead up to the top, where the cross of Jesus stands as described by the four gospels, and the remaining twenty-three books of the New Testament lead down from the top while looking back at His cross. And even among the four gospels, the Gospel of John sheds perhaps the brightest spotlight on Jesus and His cross."